Extrapolating 15min, to 10min data- help?

Hey everyone,

So I'm conducting an experiment where I'm looking at river depth and time. My river depth results are every 10 minutes, but my rain gauge is in 15 minute intervals. I'd much prefer to have the data in 10 minute intervals to improve the resolution of this particular study.

I'm struggling to find a way to extrapolate the data on excel so the data is in 10 minute intervals. I've tried changing the 15 min rain data in to 5 minutes (divide by 3, three times) but when I try to copy the formula, excel reads the adjacent rather than following the previous pattern. I've also tried =N3, =(N3-N4)*0.33333+N4, =(N4-N5)*0.66666+B5 repeated as well.

I've attached a copy of my working out (Sheet 2). If anyone could have a look I'd be very grateful!!

If you need any more information please let me know

Alex